In image forming apparatuses of the related art which use electrophotographic schemes, charging means is used to charge an electrophotographic photosensitive member (photosensitive member), and exposure means is used to perform exposure, whereby an electrostatic image is formed on the photosensitive member. The electrostatic image is developed by using developing means to form a toner image on the photosensitive member.
A contact charging scheme in which a charging member which is in contact with or close to an object to be charged is used is widely used as the charging means because an amount of ozone which is smaller than that in a corona charging scheme is produced when discharging is performed. In particular, a charging roller scheme in which a charging roller which is a roller-type charging member is used is widely used. In a typical charging roller scheme, a charging roller which is a roller-shaped charging member is in contact with the surface of a photosensitive member which is an object to be charged, and a voltage (charge voltage) is applied to the charging roller, whereby discharge which occurs in a minute gap between the charging roller and the photosensitive member causes the photosensitive member to be charged.
It is not necessary for the above-described charging member such as a charging roller to be in contact with the surface of the photosensitive member which is an object to be charged. As long as a dischargeable region defined by a gap voltage and a corrected Paschen curve is provided with certainty between a charging roller and a photosensitive member, the charging roller may be disposed in such a manner as to be close to the photosensitive member in a non-contact manner with a gap (space) of, for example, several tens of micrometers interposed therebetween. Herein, a scheme in which a charging roller is in contact with or close to an object to be charged and in which discharge which occurs in a minute gap causes the object to be charged is called a contact or close charging scheme or simply a contact charging scheme.
When a contact charging scheme is used as a charging scheme used in an image forming apparatus using an electrophotographic scheme, foreign substances, such as residual toner remaining after transfer and external additives, which have adhered to a photosensitive member adhere to a charging member, and image defects may be generated due to unevenness in charging.
A configuration is known which uses multiple charging rollers and in which a time period for which evenness in charging is maintained is prolonged in the following manner. A charging roller disposed on the downstream side is also charged so that the unevenness in charging which occurs due to the attachment of foreign substances onto a charging roller disposed on the upstream side in the rotation direction of a photosensitive member is corrected. In this case, there remains a problem in that foreign substances adhere to the charging roller disposed on the downstream side in the rotation direction of the photosensitive member when the accumulated number of image-formed sheets is increased, and in that image defects are generated due to the unevenness in charging.
PTL 1 describes that a cleaning mode is provided in which, in the case where multiple charging rollers are provided, deposits adhering to a charging roller on the downstream side in the rotation direction of the photosensitive member are removed by using a potential difference between a charge potential of a photosensitive member which is formed by using a charging roller on the upstream side and a direct voltage applied to the charging roller on the downstream side, when a non-image portion is formed.
However, in a method, as described in PTL 1, in which a charging roller is cleaned when a non-image portion is formed, cleaning needs to be performed by providing downtime during continuous image forming operations, resulting in a problem of reducing productivity of the image forming apparatus.